Call setCanAffectSystemUiFlags(false) only for PiP

It was too aggressive to call that in all cases including swipe
split-screen to home.

Video: http://recall/-/b1EZhgUkQcpML5OwZekLBN/dlG6qSf9pT2f4YlIe2Klb3
Video: http://recall/-/b1EZhgUkQcpML5OwZekLBN/cfXwlpol5vi2AER32rKu2x
Bug: 213598210
Test: 1. follow the reproduce steps in the bug. \
      2. make sure PiP from split-screen has no regressions. \
      see also the videos.
Change-Id: I33d71294595729f8cdf72258c4d45c29a77935c2